The present invention relates to a spark plug and an ignition apparatus using the same.
There is a variety types of spark plugs to ignite an engine. FIG. 11 shows one type of conventionally used ordinary spark plug. This spark plug is provided with a mounting bracket 10, a cylindrical center electrode 30 mounted on the mounting bracket 10, and a prismatic earth electrode 40 fixedly coupled with the mounting bracket 10. The center electrode 30 has one end 31 formed into a cylinder and extends from one end 11 of the mounting bracket 10, the one end 31 being insulation-supported within the mounting bracket 10 from an insulating glass member 20 intervening therebetween. The earth electrode 40 has one end secured to the one end 11 of the mounting bracket 10 and the other end extends so that its frontal surface 43 faces the one end 31 of the center electrode 30.
A high voltage generated by an ignition coil of an ignition power supply installed in an ignition apparatus is applied to a spatial gap (discharge gap) formed between the one end 31 of the center electrode 30 and the one surface 32 of the earth electrode 40. This application causes both the electrodes to ignite (i.e., spark discharge), thus firing an air-fuel mixture.
In such a spark plug, it has been known that an amount of input energy necessary for ignition is a sum of combustion energy necessary for firing the air-fuel mixture and cooling energy consumed by the electrodes of a spark plug.
Though a rate of the cooling energy to the combustion energy in the ignition energy has been unknown, to make both the center and earth electrodes compact will reduce the cooling energy, due to improvement in heat drawability of the electrodes. As a result, an amount of energy necessary for the ignition is lowered, saving energy consumed by an ignition apparatus.
However, the relationship between the conformations of the electrodes and a necessary amount of the ignition energy has not been solved yet, and how to decide the ignition energy in designing a spark plug of which electrodes are made compact has long been unknown.
Japanese Patent Laid-open Publication No. 52-362237 discloses another way of improving the ignitability of a spark plug. In the spark plug according to the publication, both of a high-voltage electrode and an earth electrode are shaped into thin types of electrodes each protruding from each support member. The inventors conducted an abrasion test on an actual spark plug produced based on the concept of the protruding electrodes disclosed by the publication. The test results proved that the electrodes wore more badly than previously supposed by the inventors. However, the foregoing publication does not provide any information about how to reduce such poor wear performance.